1. Field of the Invention
This invention relates to high-fidelity audio reproduction and more specifically to a method of enhancing low-frequency audio signals for better reproduction on small speakers.
2. Description of the Related Art
High-fidelity sound reproduction typically relies upon speakers capable of translating electrical impulses into sound waves that more or less accurately represent an original sound. Bass frequencies (for example, frequencies lower than 100 Hz) represent a particular challenge for the speaker design. To produce sounds at such bass frequencies, speaker designers have traditionally relied upon large and heavy designs (“woofers”) which are relatively expensive to produce. Woofers present both electrical and mechanical challenges for the manufacturer; they pose no less a problem for many consumers desirous of a more portable audio listening experience. In particular, headphones and portable “ear-bud” speakers have difficulties in reproducing bass frequencies without distortion and without loss in volume, sometimes severe.
Because of the difficulties reproducing bass frequencies, some audio reproduction systems have employed various means to enhance the bass response, or at least to improve the psychoacoustic perception of bass tones. In some schemes, psychoacoustic phenomena have been exploited to enhance a listener's subjective impression of bass tones. For example, U.S. Pat. No. 6,134,330 describes a known technique of enhancing the subjective experience of tones in the 40 to 100 Hz range by exploiting the phenomenon known as “virtual pitch” or “missing fundamental.” This phenomenon refers to the empirically verified fact that the presence of a series of harmonics can create the illusion of a fundamental tone at a lower frequency, where the harmonic or harmonics are at integer multiples of the (implied) fundamental frequency. This phenomenon is believed to be exploited by the cello, which is otherwise dimensionally too small to resonate in the lower range of the instrument. By adding harmonics, which are more easily reproducible with smaller transducers, one can create the impression of a bass fundamental that would be difficult to reproduce without large speakers.
As described in U.S. Pat. No. 6,134,330, it is known to filter an audio signal to select a bass subband, to generate harmonics of tones present in the bass subband, and the thereafter add said generated harmonics to the audio signal. The presence of the generated harmonics improves the perception of the low frequency portion of the audio. The generated harmonics are higher in frequency than the fundamental, and thus can be more efficiently reproduced with relatively small speakers.